Page 463 - Advanced Organic Chemistry Part B - Reactions & Synthesis

436 not act as a proton donor toward an enolate, and the enolate remains available for
subsequent reaction, as in the tandem alkylations shown below. If the saturated ketone
CHAPTER 5 is the desired product, the enolate is protonated either by use of excess proton donor
Reduction of during the reduction or on workup.
Carbon-Carbon Multiple
Bonds, Carbonyl O O O
Groups, and Other CH CH CH CH CH CH
Functional Groups Li, NH 3 CH 2 CHCH Br 2 2 2 2
2
+
1 equiv H O
2
CH 3 CH 3
CH 3
43–47% 2–2.5%
Ref. 211
H
1) Li, NH 3
O 2) n-C H I O
4 9
H 47%
H
C 4 9
Ref. 212
The stereochemistry of conjugate reduction is established by the proton transfer to
1 9
the -carbon. In the well-studied case of
-2-octalones, the ring junction is usually
trans. 213
R R
LI, NH 3
O ROH – O
H
R = alkyl or H
The stereochemistry is controlled by a stereoelectronic preference for protonation
perpendicular to the enolate system and, given that this requirement is met, the stereo-
chemistry normally corresponds to protonation of the most stable conformation of the
dianion intermediate from its least hindered side.

5.6.1.2. Dissolving-Metal Reduction of Aromatic Compounds and Alkynes.
Dissolving-metal systems constitute the most general method for partial reduction of
aromatic rings. The reaction is called the Birch reduction, 214 and the usual reducing
medium is lithium or sodium in liquid ammonia. An alcohol is usually added to serve
as a proton source. The reaction occurs by two successive electron transfer/proto-
nation steps.
H H H H H H
S H S H
Li Li
R – R R R R
. –
H H

211 D. Caine, S. T. Chao, and H. A. Smith, Org. Synth., 56, 52 (1977).
212
G. Stork, P. Rosen, and N. L. Goldman, J. Am. Chem. Soc., 83, 2965 (1961).
213 G. Stork, P. Rosen, N. Goldman, R. V. Coombs, and J. Tsuji, J. Am. Chem. Soc., 87, 275 (1965);
M. J. T. Robinson, Tetrahedron, 21, 2475 (1965).
214
A. J. Birch and G. Subba Rao, Adv. Org. Chem., 8, 1 (1972); R. G. Harvey, Synthesis, 161 (1980);
J. M. Hook and L. N. Mander, Nat. Prod. Rep., 3, 35 (1986); P. W. Rabideau, Tetrahedron, 45, 1599
(1989); A. J. Birch, Pure Appl. Chem., 68, 553 (1996).